A typical guide takes the form of a tensioner lever, which typically consists of a die-cast aluminum arm or base frame, pivotable at one end, and a resin shoe supported on the base frame and on which the transmission medium slides. The lever is pressed against the transmission medium by the plunger of a tensioner to maintain proper tension in the medium. An example of such a tensioner lever is described in U.S. Pat. No. 5,222,917, As shown in FIG. 6 of the present application, the typical tensioner lever 500 comprises a resin shoe 510 on which a timing chain slides, the shoe being mounted on a die cast aluminum base frame 520, having an I-shaped cross-section as seen in FIG. 8. The base frame 520 of the conventional tensioner lever 500 is formed in a mold, as shown in FIG. 7. The mold is composed of two parts, P1 and P2, which form an elongated mold cavity, having an I-shaped cross-section, in which the base frame is cast. These two parts, P1 and P2, are separable from each other along the direction indicated by arrows X, which is transverse to its direction of elongation of the mold cavity, but generally parallel to the upper part 521a of the mold cavity, on which the shoe-supporting surface 521 of the base frame, as seen in FIG. 8, is formed. The mold is formed with oppositely sloping drafts on opposite sides a mold parting line PL, where parts P1 and P2 of the mold meet. The drafts form a tapered configuration that facilitates opening of the mold. However, the drafts also form a central peak in the mold cavity 520′, which in turn forms a central peak in the shoe-supporting surface 521 of the base frame, as seen in FIG. 8. When the shoe is mounted on the base frame, as shown in FIG. 8, the shoe becomes unstable, and can incline toward one side or the other of the I-shaped base frame. As a result, in the operation of the guide, smooth travel of the traveling transmission medium can be impaired, the shoe can wear unevenly, and unintended disengagement of the shoe from the base frame can occur.
Because of the peaked shape of the shoe-supporting surface of the die-cast base frame as it comes out of the mold, before the shoe is attached to the base frame, the drafts on the shoe-supporting surface 521 of the base frame must be removed by cutting or machining so that the transverse cross-sections of the surface become straight. The necessity for carrying out the cutting or machining step increases the labor required to produce the guide. Moreover, since there is a possibility that the machining or cutting will not be carried out accurately, it is also possible that the shoe-supporting surface will not have the desired contour. Because the drafts produced in the process of casting the base frame must be removed by machining or cutting, more material is required, and therefore the initial weight of the casting is increased. The entire increased weight is not necessarily removed in the cutting or machining step. Furthermore, since, in the process of casting the conventional base frame 520, the mold is opened after the base frame has cooled, thermal shrinkage can occur, and loads applied during separation of the mold parts P1 and P2 can cause distortion of the casting, and also shorten the life of the mold.
This invention addresses the above-mentioned problems, and provides a guide exhibiting one or more of several advantages, including simplification or elimination of the cutting or machining step following casting, achievement of enhanced accuracy, weight reduction, stable mounting of the shoe on the base frame, reduced resistance to opening of the mold parts, and an increase in the useful life of the mold.